We have recently shown that low doses (200-300 rads) of ionizing irradiation selectively inhibit B cell responses to Class II thymus independent (TI) antigens without significantly affecting responses to Class I, TI antigens (3). We hypothesize this selective radiosensitivity is due to biochemical differences in B cell subpopulations, and may provide a new experimental approach to examine B cell development and heterogeneity in normal mice. We extended this functional analysis to mouse strains with genetic abnormalities known to affect B cell function to show genetic factors play a role in B cell resistance to ionizing irradiation. In mice which express the X-linked immunodeficiency trait xid, the entire response to TI antigens is highly radiosensitive; while in NZB mice both TI-1 and TI-2 responses are radioresistant. We propose three distinct types of experiments to determine the basis for these effects. Functional studies will: determine the kinetics of B cell recovery following irradiation; make a preliminary assessment of the genetic complexity of selective B cell radiosensitivity; and examine the contribution of microenvironment and ancillary cell types to B cell radiosensitivity. Secondly, we will define the B cell populations that are depleted by, or survive irradiation exposure, using a panel of carefully selected monoclonal antibodies to B cell surface antigens, to determine if functional changes in TI responses can be attributed to antigenically definable B cell subpopulations. Population changes will be monitored in spleen cell suspensions by flow cytometry, and in spleen microenvironments by immunohistochemistry. We will also use these techniques to monitor B cell regeneration following irradiation. Lastly, we propose to assess, preliminarily, the biochemical basis for selective B cell radiosensitivity by monitoring free radical induced plasma membrane damage, and the induction of endogenous endonuclease activity.